This invention relates to a diode safety barrier for limiting the transfer of electrical energy in one direction between a safe area and a hazardous area to a level that is too low to initiate an explosive reaction. The barriers are used for example between a control room and a processing room where there may be an explosive atmosphere of gases or dust and where there are temperature sensors, photocells or other measuring devices for monitoring conditions in the processing room and supplying process information to instruments or control apparatus in the control room.
A typical shunt diode safety barrier is essentially an energy diverting circuit comprising two forward-connected or Zener diodes or diode chains in parallel between a pair of input and output terminals, and having a current-limiting "infallible" type resistor as well as a fuse for additional protection in series with one of the terminals and the diodes whereby the diodes serve to limit the voltage transmitted and the resistor limits the current. The device is encapsulated for protection and to provide electrical insulation, and is provided with input/output pins for connection in an external circuit. The circuit may be polarized to pass signals of one polarity or non-polarized.
One of the requirements for diode safety barriers is that not only the circuits but the individual components thereof must be one hundred percent individually tested after the elements have been encapsulated in a module in order to guard against faults introduced during the encapsulation. This can be done by encapsulating the circuit completely except for leads to be used to test and confirm the operation of the elements of the circuits, after which the leads are themselves encapsulated in a further operation. This method requires two steps of encapsulation which is inherently expensive and time consuming and, once the unit is fully encapsulated, the components cannot be retested or inspected.
Another approach is as illustrated in the patent of Hawkes et al., U.S. Pat. No. 4,301,403 wherein there is disclosed a diode barrier circuit in which a low value resistor is connected between the diodes so that, by passing a sufficiently large current through one of the diodes, it can be turned off and thus allow measurements to be made on the other--the current being pulsed to avoid blowing the fuse in the barrier circuit. Testing each of the circuit components in this manner is not only difficult and relatively expensive but is also somewhat inaccurate and is limited to use with circuits having only a limited number of diodes, such as two diode chains or a star connected barrier.
The proper functioning of the barrier design is paramount since essential monitoring and measurement controls are needed in the hazardous environment. Thus, accurate testing of the components is essential. The need to quickly and accurately retest the individual components is also required for example to assist in determining the cause in the event of a failure, that is, to prove that the barrier was or was not operating properly.